The ceramic heater comprising a resistance heating element of high-melting metal as embedded between a core and an insulation sheet covering the core is in widespread use as a heating means for the automotive oxygen sensor, glow system, etc. or as a heat source for devices for gassification of petroleum oil, such as a heater for use in semiconductor heating or an oil fan heater.
FIG. 3(a) is a perspective view showing a ceramic heater of this type schematically and (b) is a sectional view taken along the line A--A of (a).
This ceramic heater comprises a cylindrical core 10, an insulation sheet 12 wrapped around said core 10 with an adhesive layer 11 interposed, and a resistance heating element 13 embedded between said core and insulation sheet, with terminal portions of said resistance heating element 13 being connected to external terminals 14 disposed externally of said insulation sheet 12 and lead wires 16 being connected to said external terminals 14, respectively.
As illustrated in FIG. 3(b), each terminal portion of said resistance heating element 13 is connected to the corresponding external terminal 14 via a plated-through hole 15 provided under the external terminal 14 in the insulation sheet 12. In this arrangement, as an electric current is applied between said external terminals 14 through said lead wires 16, the resistance heating element 13 generates heat and thereby functions as a heater.
When this heater is operated under a high temperature setting as in the above application, the resistance heating element must be caused to generate a high-temperature heat and, therefore, it is common practice to use a high-melting metal such as tungsten (W) as the material of the resistance heating element. However, there is the problem that, when used at a high temperature, a metal of this kind reacts with the surrounding ceramics to form the silicide and oxide and affect the resistance value of the heating element. Generally a ceramic heater is operated at a constant voltage and, therefore, as the resistance value of the resistance heating element is altered in this manner, the heater temperature is also affected. Such a change in heater temperature should be avoided as far as possible. Moreover, as the oxidation progresses further, the heater is degraded to suffer a problem in durability.
Therefore, it is common practice to supplement a high-melting metal with rhenium (Re) and use the alloy for the high resistance heating element to thereby control the change in resistance. Thus, Re is added to the high-melting metal such as W to reduce its reactivity with the surrounding ceramics at a high temperature and thereby control the change in resistance.
However, Re is a very expensive element and, for this reason, is a factor in the high production cost of a ceramic heater.
Moreover, in order to avoid degradation of the resistance heating element (conductor), the connecting terminals and resistance heating element proper to be formed inside of the insulation sheet are conventionally composed of an Re-containing conductor (resistance heating material) but this practice leads to a further increase in the production cost of a ceramic heater.